Abstract

Using previously developed potential energy surfaces and their couplings, non-Born–Oppenheimer trajectory methods are used to study the state-selected photodissociation of ammonia, prepared with up to six quanta of vibrational excitation in the symmetric or antisymmetric stretching modes of . The predicted dynamics is mainly electronically nonadiabatic (that is, it produces ground electronic state amino radicals). The small probability of forming the excited-state amino radical is found, for low excitations, to increase with total energy and to be independent of whether the symmetric or antisymmetric stretch is excited; however some selectivity with respect to exciting the antisymmetric stretch is found when more than one quantum of excitation is added to the stretches, and more than 50% of the amino radical are found to be electronically excited when six quanta are placed in the antisymmetric stretch. These results are in contrast to the mechanism inferred in recent experimental work, where excitation of the antisymmetric stretch by a single quantum was found to produce significant amounts of excited-state products via adiabatic dissociation at total energies of about 7.0 eV. Both theory and experiment predict a broad range of translational energies for the departing H atoms when the symmetric stretch is excited, but the present simulations do not reproduce the experimental translational energy profiles when the antisymmetric stretch is excited. The sensitivity of the predicted results to several aspects of the calculation is considered in detail, and the analysis leads to insight into the nature of the dynamics that is responsible for mode selectivity.

Received 15 December 2008Accepted 13 April 2009Published online 16 June 2009

Acknowledgments:

We are grateful to Zhen Hua Li for helpful assistance and to Hua Guo for helpful discussions. This work was supported in part by the National Science Foundation through Grant No. CHE07-04974 and in part by the United States Department of Energy Grant No. DE-AC04-94-AL85000, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.